1. Field of the Invention
The present invention relates to a process and installation for coating by electrophoresis the surface of a substrate immersed in an electrophoretic bath, and more particularly to an process and installation for coating where the bath in the vicinity of the surface is subjected to vibrational movements, particularly at sound or ultrasound frequencies.
2. Discussion of the Background
Painting by means of electrophoresis is mainly used for parts of an automobile body. The electrophoretic bath is generally comprised of an aqueous solution of a film-forming polymer material; polyepoxide type resins are widely used. An electrophoretic electric current is used to take the particles of the emulsion toward the part to be painted where they will comprise the paint layer; the electrical resistance between the part to be painted and the counter electrode increases with the thickness of the deposit.
Surface defects may be generated during this process. The surface defects of the paint layer have the form of craters which, on sheets of steel, are sites where corrosion tends to begin; in addition, in spite of the three additional layers of paint (respectively called "sealer," "base" and "varnish") which one subjects the visible parts of the vehicle body to above the cataphoresis layer, the craters remain visible and greatly degrade the appearance of these parts. These craters are present in the form of small cone-shaped holes which open onto the surface of the cataphoretic layer; they have a diameter generally between 100 and 500 micrometers at the base, between 5 and 20 micrometers at the top. These so-called "craterization" defects result from the formation of a gas, particularly hydrogen, in the vicinity of the surface area of the part during coating.
An automobile body painting unit in the traditional manner includes a container of paint and a conveyor unit for immersing the part in the bath, moving it along the bath and extracting it from the bath, as described in JP 87-268321 A, for example. The length of the container and the movement speed of the part in the container are adjusted to the thickness of the paint layer to be deposited, depending upon the paint depositing rate. The rate of depositing is proportional to the electric field in the vicinity of the part to be painted; that is, the potential difference applied between the electrode and the back electrode; with constant polarization, this speed decreases as a function of the time until it is nearly canceled when the thickness of the deposited paint layer offers a considerable electrical resistance to passage of the electrophoretic current. The part extracted from the bath is dried in order to ensure baking of the coating; for polyepoxide-type resins, the drying process lasts about 20 minutes at approximately 180.degree. C.
As described in JP 87-268321A, when one applies a paint coating in this manner onto sheets of steel coated with zinc or a zinc alloy, especially sheets of alloy galvanized steel, one will observe surface defects ("pinhole gases") on the layer of paint, which result from the formation of gas bubbles on the surface to be painted during electrical deposition. In order to prevent the formation of these defects, JP 87-268321A proposes that one can subject the electrophoretic bath to vibrational movements at ultrasound frequencies during the passage of the electrophoretic current.
In order to produce vibrations in the bath, one immerses ultrasound-emitting generators in the bath along the movement path of the part, on either side of the part; these ultrasound emitters are distributed on either side of the movement path along two longitudinal walls of the paint container (reference numeral 7 in FIGS. 1 and 2 of JP 87-268329A) and are connected to an adjustable power supply device. This ultrasound electrodeposition process is expensive because it requires the installation of many emitters along the movement path of the parts.